1. Field of the Invention
The present invention relates to a zoom lens system which is suitable for use in a broadcasting television (TV) camera, a video camera, a digital still camera, and a silver-halide camera, and also to an image pickup apparatus including the zoom lens.
2. Description of the Related Art
In recent years, there have been demanded a zoom lens system having a high zoom ratio and high optical performance for image pickup apparatus such as a television (TV) camera, a silver-halide camera, a digital camera, and a video camera. A positive lead and telephoto type four-unit zoom lens system in which four lens units are provided in total and one of the lens units located closest to an object side has a positive refractive power has been known as the zoom lens system having a high zoom ratio. For instance, there is known a four-unit zoom lens system which includes a first lens unit having a positive refractive power for focusing, a second lens unit having a negative refractive power for magnification, a third lens unit having a negative refractive power for correcting image plane variation, and a fourth lens unit having a positive refractive power for image formation. As to this four-unit zoom lens system, there is known a four-unit zoom lens system in which an optical material having an extraordinary dispersion characteristic is used, so that chromatic aberration is corrected appropriately and a high optical performance is provided (see U.S. Pat. Nos. 6,825,990 and 6,141,157).
U.S. Pat. No. 6,825,990 discloses a structure in which a first sub lens unit which is fixed during focusing in a first lens unit which is fixed during zooming includes two negative lenses. As to the two negative lenses, a low dispersion material is used as a material of the negative lens disposed on the side closest to an object, so that lateral chromatic aberration is corrected appropriately mainly at the wide angle end. U.S. Pat. No. 6,141,157 discloses a structure in which a thin resin layer having a meniscus shape and a negative refractive power is formed on a lens surface of a second or third positive lens counted from the object side in a first lens unit which is fixed during zooming. Utilizing this resin layer, higher order lateral chromatic aberration is corrected appropriately mainly on the wide angle side without an increase in size or weight.
A positive lead type four-unit zoom lens system having a structure described above may support a high zoom ratio relatively easily. In order to obtain high optical performance in this four-unit zoom lens system, it is important to correct lateral chromatic aberration at the wide angle end and longitudinal chromatic aberration at the telephoto end appropriately. It is easy to correct the chromatic aberration appropriately including the lateral chromatic aberration and the longitudinal chromatic aberration if an optical material having an extraordinary dispersion characteristic is used. However, it is difficult to correct the chromatic aberration appropriately by simply using a lens made of the optical material having an extraordinary dispersion characteristic. In particular, in order to obtain high optical performance over the entire zoom range in the four-unit zoom lens system described above, it is an important factor to set appropriately a material of each lens included in the first lens unit which does not move for zooming. For instance, in order to correct a secondary spectrum of the longitudinal chromatic aberration appropriately on the telephoto side, it is important to set an appropriate difference between dispersions of materials of the positive lens and the negative lens in the first lens unit. If this difference is set inappropriately, it is difficult to suppress higher order aberration mainly.